"That's obviously something that is disappointing to lose," said the third-seeded Scot, who smashed his racquet in frustration after losing the second set.

"That's many years' work that's gone into building that sort of consistency.

"Also to lose a match like that, that was over four hours, tough obviously after a couple of tough matches earlier in the tournament, as well. It's a hard one to lose, for sure."

The defeat came at Louis Armstrong Stadium, the US National Tennis Center's second court.

"That court is a lot quicker than Ashe (Arthur Ashe Stadium). I felt like I was on the back foot quite a lot. Wasn't able to play that offensively," said Murray.

"But when you're playing against someone that has the game style that he does, you're always going to have to do a fair bit of defending, especially if he serves well."

Murray lamented some lost chances against the 2.03m tall South African, seeded 15th.

"When you're playing against players that are at that level, you need to make them think and then give them a chance to get nervous," he said, about how the match might have gone differently had he cashed in on a couple of break points.

"I felt like I had my opportunities there but didn't manage to capitalise on them." - Reuters.